This invention relates to polymerization catalyst systems and polymerization processes.
Supported chromium oxide catalyst systems long have been used to prepare olefin polymers in hydrocarbon solution or slurry processes to give products having excellent characteristics from many standpoints. A number of supports have been broadly disclosed in the art for the support of chromium oxide catalyst systems including, silica, alumina, boria, zirconia, silica-alumina, and other refractory metals. In order to obtain a polymer product with easy processing characteristics, catalyst systems preferably have a low pore volume and high surface area. Generally, it is recognized that catalyst systems having a low pore volume and high surface area can result in a higher melt index and produce an olefin polymer that is easier to process. Unfortunately, it also has been found that decreasing the catalyst system pore volume usually corresponds to a decrease in the catalyst system surface area. These catalyst systems provide polymers with extraordinarily good properties which can only be achieved presently on the commercial scale by making bimodal polymers from two reactors. Thus the first benefit is one of economics. With the magnesia modified silica catalyst we get polymers with fabulous properties more cheaply than our competitors. For our HMW film it is necessary to add a fluoropolymer to process the resin when it is blown into film. So processing is not the real forte for this particular catalyst system. These characteristics are contrary to what is desired for polymer processability.
It is also known that catalyst systems need to be dry for best polymerization productivity and activity. Therefore, catalyst systems are heated and dried prior to use. Unfortunately, heating of catalyst systems can cause problems with catalyst system integrity. For example, if a catalyst system uses a silica-based inorganic oxide support, heat can cause the silica to melt, or sinter, and therefore decrease the surface area of the resultant catalyst system. If alumina is selected as a support material, heating of the alumina can cause the alumina to fracture and create uneven and rough catalyst system particulates which can result in a polymer product that is difficult to handle.
Therefore, it is an object of the invention to provide an improved silica-containing inorganic oxide catalyst system supports.
It is yet another object of this invention to provide a silica-containing inorganic oxides having a high surface area.
It is a further object of this invention to provide novel silica-containing inorganic oxides that either retain or increase surface area upon heating.
It is a further object of this invention to provide an improved chromium catalyst system.
It is yet another object of this invention to provide a catalyst system suitable for use in polymerization processes.
It is yet another object of this invention to provide a chromium catalyst system that can produce an olefin polymer having a decreased melt index.
In accordance with one embodiment of this invention a process is provided to prepare a magnesium treated silica-containing composition comprising contacting a silica-containing inorganic oxide with a magnesium-containing compound and converting said magnesium-containing compound to a magnesium oxide to produce a magnesium treated silica-containing composition.
In accordance with another embodiment of this invention a process is provided to contact said magnesium treated silica-containing composition with a chromium compound to produce a catalyst system composition.
In accordance with yet another embodiment of this invention a polymerization process is provided using a catalyst system comprising chromium supported on a magnesium treated silica-containing composition.
In accordance with the present invention, a predominately silica-containing compound is contacted with at least one magnesium compound convertible to the oxide form, and then calcined. The essence of this invention is either precipitating, or doping, magnesium within the pores of a silica-containing compound. Magnesium treatment of the silica-containing compound can be done in accordance with different embodiments of the invention, discussed in detail below. The resultant magnesium treated silica-containing compound can be used as a catalyst system support. As used in this disclosure, the term xe2x80x9csupportxe2x80x9d refers to a carrier for another catalytic component. However, by no means, is a support necessarily an inert material; it is possible that a support can contribute to catalytic activity and selectivity.
Silica-containing compounds employed to prepare polymerization catalyst systems of the present invention must contain a major proportion of silica. Preferred silica-containing compounds contain a substantial proportion of silica, e.g., at least about 50% by weight of silica, preferably at least about 70%, and most preferably 90%, by weight of silica, although still larger proportions of silica can be used. The preferred predominantly silica-containing compounds of the present invention consist essentially of less than about 50% by weight of at least one additional metal oxide such as, for example, alumina, boria, magnesia, titania, zirconia and mixtures of any two or more thereof. Generally, the silica-containing compound employed has a surface area, prior to magnesium treatment, of at least about 10 square meters per gram (m2/g). Preferably, the initial surface area of the silica-containing compound, prior to treatment with a magnesium compound, can be at least 50 m2/g, and most preferably, any silica-containing compound employed is a high surface area silica, i.e., support, with a surface area in excess of about 100 m2/g.
In accordance with one embodiment of this invention, a magnesium salt can be used in a very concentrated aqueous solution and precipitated within the pores of a silica-containing compound. Exemplary magnesium salts include, but are not limited to, magnesium nitrate, magnesium chloride, magnesium acetate, and mixtures thereof The concentrated aqueous solution of the magnesium salt is mixed with the silica-containing compound. Mixing can occur by spraying the aqueous solution containing the magnesium salt onto the silica-containing compound, or by slurrying the aforementioned components together. The concentration of the aqueous magnesium solution can be any amount sufficient to deposit enough magnesium into the pores of the silica-containing compound to increase the surface area of the silica-containing compound and yet, not significantly decrease the pore volume of the silica-containing compound. Usually, concentrations of about 125 millimoles of the magnesium salt per 100 g (mmol/100 g) of support are sufficient. Preferably, concentrations within a range of about 200 to about 700 mmol/100 g of support and most preferably, within a range of 250 to 400 mmol/100 g of support, are preferred. Higher or lower magnesium salt concentrations do not have significant, beneficial effects on surface area.
Contacting conditions of the silica-containing compound and the magnesium compound are not critical. Any temperature and any period of time can be suitable. For convenience, contacting generally is carried out at about room temperature, although higher or lower temperatures, within a range of about 40xc2x0 F. to about 100xc2x0 F., can be used. A time period sufficient to allow the support and magnesium compound to come into intimate contact is all that is necessary. Thus, the silica-containing compound and the magnesium salt solution can be brought into contact for as little time as a few seconds to several hours or more, such as, for example, about 5 seconds to about 24 hours, as convenient.
After sufficient contact time, any basic compound can be added to the magnesium/silica-containing compound slurry to precipitate the magnesium within the pores of the silica-containing compound. Exemplary basic compounds include, but are not limited to hydroxides, such as for example, sodium hydroxide, potassium hydroxide, ammonium hydroxide, and mixtures thereof Preferably, the basic compound is ammonium hydroxide (NH4OH), due to availability and ease of use. Heat and increased times can increase effectiveness of introducing and/or precipitating magnesium into the pores of the silica-containing compound. Optionally, the magnesium treated silica-containing support can be heated to temperatures within a range of about 30xc2x0 C. to about 90xc2x0 C. and aged for times of up to eight hours. Preferably a temperature range of 50xc2x0 C. to 60xc2x0 C. and an aging time of about one hour is sufficient.
Following precipitation of magnesium within the pores of the silica-containing compound, a substantial portion of excess liquid can be removed by any suitable means, such as, for example, decantation, filtration, or any other method known in the art.
After removal of excess liquid, the magnesium-treated silica-containing compound is rinsed with a dilute basic solution, such as, for example, ammonium hydroxide and washed with deionized. The solid then is dried in any manner known in the art to remove any residual liquid. Any suitable means can be employed, such as, for example, oven drying, vacuum drying, alcohol washing, and/or passing of vigorous steam of dry (moisture-free) gas over the magnesium treated silica-containing compound. Preferably, for ease of use, the magnesium treated silica-containing compound is washed with an alcohol. Any alcohol that can remove residual liquid, i.e., water, can be used to wash the magnesium-treated silica-containing compound. Exemplary alcohols include, but are not limited to, methanol, ethanol, propanol and mixtures thereof.
The partially dried, magnesium-treated silica-containing compound then is further dried to remove any remaining absorbed solvent and subjected to calcination conditions. Calcination can be conducted by heating the magnesium treated silica-containing compound in the presence of a dry, oxygen-containing gas, such as, for example, air, under conditions sufficient to convert any magnesium to an oxide. Generally, temperatures within a range of about 300xc2x0 C. to about 800xc2x0 C., for a time within a range of about 0.5 to about 20 hours are sufficient. Typically, less time is required at higher calcination temperatures and more time is required at lower calcination temperatures. Preferably, heating occurs at a temperature of around 350xc2x0 C. to about 450xc2x0 C. (about 662xc2x0 F. to about 842xc2x0 F.) for a period of two to four hours, depending upon the amount of compound being calcined, and then raised to a temperature in the range from 400xc2x0 C. to 800xc2x0 C. and held and this temperature from 3 to 15 hours depending on the amount of compound being calcined.
In accordance with a second embodiment of this invention, magnesium treatment can comprise coprecipitating magnesia with a silica-containing compound. This coprecipitation method can be as equally effective in increasing the surface area of silica containing compounds. As used in this disclosure, coprecipitation is preparation of a catalyst system wherein a magnesium compound is coprecipitated with a silica-containing compound and then chromium can be added Another method of coprecipitation can be preparation of a catalyst system wherein a magnesium compound is coprecipitated with a silica-containing compound and a chromium compound.
The magnesium treated silica-containing support, after calcination, must contain a chromium compound in order to produce a polymerization catalyst system. The chromium compound can be added in accordance with any method known in the art. Exemplary methods include starting with a silica/chromium cogel-containing compound and then treating with a magnesium-containing compound. Another exemplary method is to add a chromium compound with the magnesium salt and precipitate both chromium and magnesium within the pores of the silica-containing compound. Another exemplary method is embodiment of the invention comprises adding chromium in conjunction with an alcohol solution prior to calcinations of the silica-containing support. A further embodiments of this invention is to add the chromium after the support has been calcined. Addition of chromium also can be done after the magnesium-treated silica-containing compound has been dried by combining additional magnesium with a solution of a chromium compound.
Commonly used polymerization cocatalysts can be used, if desired, but are not necessary. Exemplary cocatalysts include, but are not limited to, metal alkyl, or organometal, cocatalysts, i.e., alkyl boron and/or alkyl aluminum compounds. The term xe2x80x9cmetalxe2x80x9d in organometal is intended to include boron. Often these cocatalysts can alter melt flow characteristics (melt index or high load melt index) of the resultant polymer. While not wishing to be bound by theory, it is believed a cocatalyst can act as a scavenger for catalyst system poisons.
If the cocatalyst is an alkyl boron compound, trihydrocarbylboron compounds are preferred and trialkyl boron compounds are most preferred. Preferably, the alkyl groups have from about 1 to about 12 carbon atoms and preferably, from 2 to 5 carbon atoms per alkyl group. Trialkyl boron compounds, such as, for example, tri-n-butyl borane, tripropylborane, and triethylboran (TEB) are preferred cocatalysts because these compounds are effective agents to improve polymer properties, such as, for example, to reduce melt flow and retard polymer swelling during polymerization. Other suitable boron compounds include trihydrocarbyl boron compounds broadly; triaryl boron compounds, such as, for example, triphenylborane; boron alkoxides, such as, for example, B(OC2H5)3; and halogenated alkyl boron compounds, such as, for example, B(C2H5)Cl2. By far, the most preferred cocatalyst is triethylboran, for the reasons given above.
Other suitable cocatalysts can be are aluminum compounds of the formula AlRxe2x80x2nX3xe2x88x92n, where X is a hydride or halide, Rxe2x80x2 is a 1 to 12 carbon atom hydrocarbyl radical and n is an integer of 1 to 3. Triethylaluminum (TEA) and diethylaluminum chloride (DEAC) are particularly suitable.
A cocatalyst, when used, usually can be used in an amount within a range of about 1 to about 20 parts per million (ppm), or milligrams per kilogram (mg/kg), based on the mass of the diluent in the reactor. Preferably, cocatalyst is used in an amount within a range of 1 to 12 mg/kg, for cost effectiveness and best resultant polymer properties. Expressed in other terms, a cocatalyst can be present in an amount so as to give an atom ratio of cocatalyst metal to chromium within a range of about 0.5:1 to about 10:1, preferably 2:1 to 8:1.
The cocatalyst either can be premixed with a catalyst system or added as a separate stream to the polymerization zone, the latter being preferred.
The polymers produced in accordance with the process of this invention are homopolymers of ethylene and copolymers of ethylene and higher alpha-olefin comonomers. Preferably, the ethylene concentration in the polymerization reactor is within a range of from about 2 weight percent to about 20 weight percent, based on the total liquid contents of the reactor. Most preferably, the ethylene concentration in the polymerization reactor is within a range of from about 4 to about 15 weight percent. Measured in another manner, ethylene concentration in the polymerization reactor flash gas is within a range of from about 5 weight percent to about 12 weight percent. Most preferably, the ethylene concentration in the polymerization reactor flash gas is within a range of from about 6.5 to about 10 weight percent. While ethylene concentration does not significantly affect the molecular weight of the resultant polymer, higher or lower ethylene concentration can effect catalyst activity.
The alpha-olefin comonomers used in the present invention must be selected from the group consisting of 1-butene, 1-hexene, and mixtures thereof in order to produce a copolymer with desirable properties as well as ease of use in a loop/slurry polymerization reaction process. The most preferred comonomer is 1-hexene to produce a copolymer with the best product properties. If a comonomer is present during polymerization, the comonomer concentration in the polymerization reactor is within a range of from about 0.5 to about 20 mole percent. Most preferably, comonomer is present within a range of about 1 to about 15 weight percent. As used in this disclosure the term xe2x80x9cpolymerxe2x80x9d includes both homopolymers and copolymers.
Polymerization can be carried out in any manner known in the art, such as gas phase, solution or slurry conditions, to effect polymerization. A stirred reactor can be utilized for a batch process, or the reaction can be carried out continuously in a loop reactor or in a continuous stirred reactor.
A preferred polymerization technique is that which is referred to as a particle form, or slurry, process wherein the temperature is kept below the m temperature at which polymer goes into solution. Such polymerization techniques are well known in the art and are disclosed, for instance, in Norwood, U.S. Pat. No. 3,248,179, the disclosure of which is hereby incorporated by reference.
The preferred temperature in the particle form process is within a range of about 185 to about 230xc2x0 F. (about 85 to 110xc2x0 C.), although higher or lower temperatures can be used. Two preferred polymerization methods for the slurry process are those employing a loop reactor of the type disclosed in Norwood and those utilizing a plurality of stirred reactors either in series, parallel or combinations thereof wherein the reaction conditions can be different in the different reactors.
The molecular weight of the polymer can be controlled by various means known in the art such as adjusting the temperature (higher temperature giving lower molecular weight) and introducing, or varying the amount of, hydrogen to alter the molecular weight, or varying the catalyst compounds.
Polymers produced in accordance with this invention have increased density, broadened molecular weight distribution especially on the high molecular weight end, decreased MI, and decreased HLMI, as compared to polymers prepared from conventional supported chromium catalyst systems be they chromium on silica or chromium on a silica-titania matrix. It is believed that the uniqueness of this catalyst system is derived from the formation of a catalyst with a large surface area and unique polymerization sites. These characteristics can produce a polymer, or resin, which has an increase in the molecular weight region which promotes good properties in resins while not enhancing the very high molecular weight tail or long chain branching which characterizes chromium based catalysts. While not wishing to be bound by theory, it is believed that polymers produced in accordance with this invention are unique in that the polymer chains are intertwined in each polymer particle; each polymer particle can be considered xe2x80x9call-inclusivexe2x80x9d as to polymer characteristics. This catalyst system composition most preferably is applicable for use with ethylene polymerization.
The resultant ethylene polymer usually can have a density within a range of about 0.91 to about 0.975 g/cc, and preferably within a range of about 0.945 to about 0.96 g/cc. The polymer melt index (MI) usually is within a range of about 0.015 to about 0.7 g/10 min and preferably within a range of about 0.02 to about 0.5 g/10 min. The polymer high load melt index (HLMI) of the resultant polymer usually is within a range of about 1 to about 175 g/10 min and preferably within a range of about 4 to about 70 g/10 min. The shear ratio (HLMI/MI) is usually within a range of about 40 to about 250, and preferably within a range of about 50 to 200. Polymers having characteristics within the given ranges are especially useful for applications of blow molding and/or film production.
The uniqueness of the disclosed and claimed catalyst system is the ability to make polymers with a molecular weight distribution (MWD) that promotes good properties particularly dart impact in high molecular weight (HMW) film applications. While not wishing to be bound by theory, it is believed that these property improvements result from the lessening of long chain branching (LCB) and/or the reduction in HMW tails as a result of the increased surface area.
A further understanding of the invention and its advantages is provided by the following examples.